Patients with known or suspected psoriatic arthritis are evaluated at the Clinical Center. Studies include characterization of the clinical and laboratory features of the disease, research studies of the natural history and pathogenesis, genetic studies, and determination of the patients eligibility to enter experimental therapeutic protocols. Patients are asked to contribute blood, skin or synovial membrane samples for immunologic and genetic research studies. Immunologic studies include investigations of 1) the cytokine profile of the disease (type 1 vs type 2 immune response); 2) the role of the interaction of T cells with antigen presenting cells; and 3) the regulation of apoptosis of keratinocytes and synoviocytes. Genetic studies include establishment of pedigrees with multiplex affected families and genetic linkage analysis.